Process for the catalytic hydroisomerization of crude normal-pentane



July 31, 1962 N. L. CARR 3,047,645

PROCESS FOR THE CATALYTIC HYDROISOMERIZATION OF CRUDE NORMAL-PENTANE Filed Dec. 7, 1959 p. s i. :1.

REACTION TEMP. "F.

INVENTOR.

NORMAN L. CARR BYZW S j ATTORNEY rates This invention comprises a novel process for the catalytic hydroisomerization of a crude n-pentane cut containing a small amount of other closely related alkanes and naphthenes. In particular, the invention is concerned With an isomerization process which effects a maximum isomerization rate for n-pentane, while maintaining a substantially zero fouling rate or aging rate for the catalyst used.

According to this invention, it has been found that the isomerization of a pentane feed stock containing up to about vol. of hydrocarbon impurities consisting predominantly of hexanes and cyclopentane can be carried out at high reaction rate and low catalyst aging rate by processing the pentane feed and hydrogen in the presence of a catalyst consisting of 0.5-0.75% wt. palladium on small silica-alumina (containing 70-90% silica) pellets at a hydrogen partial pressure of 375-425 p.s.i.a. and a hydrocarbon partial pressure and a reaction temperature which lie within the shaded area of the drawing.

Isomerization has recently come into prominence as a unit process in the petroleum industry for increasing the octane number of low-molecular-weight, liquid aliphatic hydrocarbons. In particular, the isomerization of n-pentane and n-hexane has been extensively investigated. The isopentanes and isohexanes have substantially increased octane numbers over the corresponding normal paraffins. In the copending patent application of Hillis O. Folkins et al., Serial No. 691,996, filed October 23, 1957, there is described an isomerization process which utilizes a solid refractory catalyst consisting of a small amount of palladium supported on an acidic silica-alumina hydrocarbon cracking catalyst. In that patent application, the process is described as being applicable to the isomerization of n-C -C hydrocarbons at temperatures below 800 F. and being highly selective in the conversion of n-pentane and n-hexane to their respective isomers. Folkins et al. describe their isomerization process in detail and set forth specific ranges of conditions of temperature, pressure, space velocity, and hydrogen/hydrocarbon mol ratio, which are desirable for optimum yields of the C -C isomers. If the thermodynamic data for the formation of various hydrocarbon isomers are examined, it is seen that the formation of branched-chain C4C'1 alkanes is favored at lower temperatures. Thus, theoretically the maximum yield per pass for formation of branched-chain isomers of the lower alkanes should be obtained at room temperature. It has been found, however, that the conversion of normal parafiins to isoparaffins requires a moderately high temperature and a suitable catalyst if substantial yields of isoparaffins are to be obtained. The palladium on silica-alumina catalysts are effective in the isomerization of n-pentane and n-hexane at temperatures of the order of 650800 F. At higher temperatures, there is a tendency toward hydrocracking and aromatization in preference to the isomerization reaction, while at lower temperatures the isomerization reaction rate is very slow. It has thus become important to determine the conditions which are conducive to a high rate of isomerization without excessive promotion of undesired side reactions, such as hydrocra-cking and aromatization. It has also been found that at elevated temperatures, while there is a substantial increase in the rate of the isomerization reaction, there is a substantial tendency toward coke red fouling of the catalyst and a general decline in catalytic activity on extended use. It is therefore necessary to balance the requirement of high reaction rate against the disadvantages of a high aging rate (rate of decline of activity) for the catalyst. In the Folkins et al. application, data are presented which define an excellent process for isomerization of a substantially pure n-paraifin hydrocarbon, and it is indicated that mixtures of hydrocarbons could be isomerized satisfactorily at process conditions which represent compromise values between the optimum values for the individual hydrocarbons. It has been found, however, that mixtures of hydrocarbons can be isomerized under the general conditions set forth in Folkins et al. application only at temperatures less than about 740 R, which do not result in coke fouling and excessive aging rate for the catalyst. When a hydrocarbon cut is used, which corresponds to a particular hydrocarbon fraction, such as n-pentane or n-hexane, the cut usually' contains up to about 10% vol. of other hydrocarbons as impurities, particular naphthenic hydrocarbons and the adjacent parafiin hydrocarbons. Thus, a n-pentane cut used for the feed to an isomerization process would ordinarily contain up to 10% vol. of hydrocarbon impurities consisting essentially of n-hexane and cyclopentane. The inclusion of these hydrocarbon impurities in the n-pentane feed introduces problems into the isomerization process for which there were previously no solutions. The inclusion of an appreciable amount of naphthenic hydrocarbons, such as cyclopentane and cyclohexane in an isomerization feed usually results in a rapid coke fouling and high aging rate. The catalyst aging rate is also affected by the catalyst pellet size and the palladium metal content.

I have previously shown in my paper Kinetics of Catalytic isomerization of n-Pentane, read before the Division of Petroleum Chemistry, American Chemical Society, April 1959 meeting, that for any specific combination of catalyst, reaction temperature, and feed stock composition, there is an optimum total pressure at which maximum isomerization rate is obtained at each hydrogen/hydrocarbon ratio. However, many of the maximum isomerization reaction rates are accompanied by excessively high catalyst fouling rates, because, in general, both rates are increased by increasing the same reaction variables, viz., temperature, hydrocarbon partial pressure, molecular weight of the feed stock, and naphthene concentration in the feed stock.

It is therefore one object of this invention to provide a new and improved process for the isomerization of an impure hydrocarbon feed which consists predominantly of n-pentane containing a small amount of other hydrocarbons as contaminants.

A further object of this invention is to isomerize a n-pentane feed containing up to 10% vol. of hydrocarbon impurities consisting essentially of hexanes and cyclopentane, using a catalyst consisting of 0.5-0.75% wt. palladium on an acidic silica-alumina support, under conditions of temperature, pressure, and hydrogen/hydrocarbon ratio which effect a maximum isomerization reaction rate at a substantially zero aging rate for the catalyst.

Another object of this invention is to provide an improved catalytic isomerization process in which the decline of catalyst activity is elfectively mitigated, while maintaining very high reaction rates.

A feature of this invention is the provision of an improved isomerization process using an isomerization catalyst consisting of 0.5-0.75% wt. palladium on an acidic silica-alumina (70-90% wt. silica) support in which the reaction temperature and partial pressures of pentane and hydrogen are carefully controlled to provide a maximum isomerization reaction rate at a substantially Zero aging rate for the catalyst.

Another feature of this invention is the provision of an improved isomerization process in which a sulfur-free hydrocarbon feed consisting of n-pentane containing up to about 10% vol. of hydrocarbon impurities consisting essentially of hexanes and cyclopentane is isomerized by contact with an isomerization catalyst consisting of 0.30.7% Wt. palladium supported on silica-alumina (containing 70-90% wt. silica), at a hydrogen pressure of 375-425 p.s.i.a., hydrocarbon pressure of about l30-175 p.s.i.a., and a reaction temperature of about 775-790 F., all lying Within the shaded area of the drawing.

Still another feature of this invention is the provision of an improved isomerization process using a palladium on silica-alumina catalyst in which catalyst pellets are used of a size and metal content which are highly resistant to aging and have a high activity for formation of isoparaffins.

Other objects and features of this invention Will become apparent from time to time throughout the specification and claims as hereinafter related.

In the accompanying drawing, to be taken as part of this specification, there is presented a graph showing the range of isomerization reaction temperature and partial pressure of the crude n-pentane feed at a hydrogen partial pressure of 375-425 p.s.i.a., which results in a maximum isomerization reaction rate (K=812) and a substantially zero aging rate for the catalyst (0.50.75% wt. palladium on an acidic silica-alumina support).

This invention consists essentially of a process for isomerization of an impure n-pentane feed containing up to about 10% vol. of hydrocarbon impurities such as hexanes and cyclopentane. The n-pentane feed treated in this process is first subjected to a desulfurization process, such as catalytic dehydrosulfurization, optionally including causticand water-washing and drying of the efiiuent, or stabilization, to reduce the sulfur content to the range of about 5-50 ppm. IThis hydrocarbon feed is heated to an elevated temperature, preferably about 400800 F. (although higher temperatures may be used), and passed through a desulfurization reactor or guard case containing a suitable desulfurizing reactant to fix and remove sulfur without the release of hydrogen sulfide. Desulfurizing catalysts, which are well known in the prior art for removing sulfur (as hydrogen sulfide) from hydrocarbons, include various metals, such as copper, nickel, iron, molybdenum, and cobalt, and their oxides and various compounds thereof, such as copper molybdate, cobalt molybdate, nickel molybdate, etc., preferably supported on a silica, silicaalumina, or alumina support. Such materials are also effective as reactants in a guard case to fix and remove sulfur without the release of hydrogen sulfide. The desulfurization catalyst may be the same as the catalyst used in the isomerization reaction. Additonal details of this desulfurization process and the need for using a totally desulfurized feed are discussed in my copending application, Serial No. 731,778, filed April 29, 1958, now Patent No. 2,951,888. This chemical treatment usually reduces the sulfur content of the feed to less than about 1 ppm. and may produce a sulfur content of practically zero. The completely desulfurized hydrocarbon feed is then passed to the isomerization reactor (containing the 0.5-0.75% palladium on silica-alumina catalyst), with free hydrogen at a partial pressure of 375425 p.s.i.a., and at a reaction temperture in the range of about 775 790 LR, and pentane feed partial pressure in the range of about 130-175 p.s.i.a., all lying within the shaded area of the drawing. Under these conditions of reaction, the n-pentane is converted to isopentane in a very high yield, approaching equilibrium, and at a very high reaction rate (rate constant K=812) and at a catalyst aging rate of substantially zero. iAging, as it is used herein, means a change in catalyst activity with processing time under the test conditions used, and includes all possible poisoning action from coke, trace sulfur, and other forms of activity changes which might be dependent upon the basic catalysts or system. The process conditions for maximum isomeri- Zation reaction rate accompanied by a substantially zero catalyst aging rate for an impure n-pentane feed, as previously described, using a catalyst consisting of 0.5O.75 wt. palladium on silica-alumina (containing 70-90% Wt. silica) lie substantially along the curved line AB (or surface, since the line extends along the Z axis in the range from 375-425 p.s.i.a hydrogen partial pressure in the attached drawing). Along the line (or surface) AB, the isomerization rate constant is at a maximum value (in the range from 8 to 12) and the catalyst aging rate is substantially Zero. A substantial increase in hydrogen partial pressure above 425 p.s.i.a. has been found to increase the aging rate and decrease the isomerization rate, while a substantial decrease in hydrogen partial pressure below 375 p.s.i.a. increases the aging rate but also increases the isomerization rate. A decrease in hydrocarbon partial pressure below 130 p.s.i.a. decreases the isomerization rate substantially without significantly affecting the aging rate, while an increase in hydrocarbon partial pressure above the limits defined by the line (or surface) AB increases both isomerization rate and catalyst aging rate. A decrease in the reaction temperature below 775 F. decreases the isomerization rate very rapidly without aifecting the catalyst aging rate substantially, while increasing the temperature above the limits defined by the line (or surface) AB increases isomerization reaction rate, but also increases the catalyst aging rate.

The catalysts containing 0.5-0.75% wt. palladium on silica-alumina (7090% silica) are more active than catalysts containing less palladium and are more resistant to aging. Catalysts containing a higher proportion of palladium are not appreciably more active and are actually less selective for formation of the desired isoparaffins. Catalysts which are formed or broken into pellets are much more resistant to aging than A" or A3" catalyst pellets. Smaller catalyst pellets are impractical as they tend to be entrained by the feed at high flow rates and also increase the resistance of the reactor by a substan tial amount.

The following non-limiting examples are illustrative of the scope of this invention.

EXAMPLE I A 0.65% wt. palladium on 87/ 13 silica-alumina catalyst pellets) was prepared by impregnating an 87/13 silica-alumina hydrocarbon cracking catalyst with an acidified solution of palladium chloride sufficient to produce the desired concentration of palladium metal. The impregnated catalyst thus produced was dried and reduced with hydrogen at -a temperature of 750 975 F. to produce a highly active catalyst of the desired composition. This procedure for the preparation of the catalyst is described in considerable detail in the aforementioned copending application of Hillis O. Folkins et al. The catalyst which was thus prepared was used in carrying out a number of experimental tests for catalyst aging under different reaction conditions. In each case the hydrocarbon feed consisted of 88% vol. n-pentane, 2% vol. isopentane, 5% vol. cyclopentane, and 5% vol. n-hexane. The cyclopentane concentration used was slightly higher than would be encountered in a n-pentane feed, but represented the normal level which would be encountered in an isomerization reactor as a result of cyclopentane build-up produced by recycling. The hydrocarbon feed was desulfurized to a sulfur content less than about 1 ppm, as described in my copending patent application, using a guard-case containing a desulfurization reactant consisting of 15% reduced nickel molybdate on /25 silica-alumina. The desulfurized impure n-pentane feed was circulated over the catalyst in an isomerization reactor for extended periods of time, up to about hours, at different reaction conditions of pressure, hydrogen/hydrocarbon ratio, and reaction temperature. In each experiment, the yield of isopentane was noted initially and after an extended period of time to determine the aging rate of the catalyst, which is "are.

expressed in decrease of yield percent, per 100 hours of process operation. The reaction rate constant for the isomerization process under the conditions used as calculated according to the equation:

K: (LWHSV) In I: 1

where LWHSV is the liquid weight hourly space velocity and x is the percent isopentane yield. The rate constant K provides a basis for comparison of catalyst activity under different conditions of temperature, space velocity,

etc.

tial pressure was 170 p.s.i.a. The initial yield of isopentane 0 was 54.4% and the final yield was 52.5%. The reaction rate constant for the process under these conditions is 11.8, while the aging rate for the catalyst is 2.0% decline per 100 hours of process operation.

A number of additional runs of the same type were carried out in which the temperature and the partial pressures of hydrogen and the pentane feed were varied and determinations were made of the rate constant K and the aging rate for the catalyst under the isomerization conditions used. These data were obtained in a planned experimental program based on the so-called steepest ascent technique which enabled me to determine the conditions at which maximum isomerization rate is obtained together with a substantially zero catalyst aging rate. In Table I there are set forth in tabular form the rate constants and aging rate for different hydrogen and pentane partial pressures and ditferent isomerization reaction temperatures These data are analyzed mathematically according to the steepest ascent technique and are expressed graphically in the drawing which shows the shaded area of temperature and hydrocarbon partial pressure as being the area of maximum isomerization reaction rate and substantially zero catalyst aging rate. This shaded area (which should actually be considered a volume since it extends along the Z aXis in the range from 375 to 425 p.s.i.a., hydrogen partial pressure) represents the region in which the isomerization rate constant lies in the range from 8 to 12 and the catalyst aging rate is substantially Zero. In Table I, from which the drawing is derived, the reaction rate constant K is shown in the left of each column and the aging rate is set forth in parenthesis, ex pressed as decline in yield percent per hours of process operation.

EXAMPLE II When an isomerization catalyst is prepared consisting of 0.65% wt. palladium on 75/25 silica-alumina pellets) and used in the isomerization of an impure npentane feed stock, as above described, the maximum rate constant for the reaction is slightly less than the catalyst using 87/ 13 silica-alumina support. When this catalyst is used in the isomerization of the impure npentane feed under different conditions of reaction temperature, and partial pressures of hydrogen and pentane, it is found that the reaction rate is at a maximum and the aging rate is substantially zero when the process conditions are maintained within the range specified in the shaded area of the drawing. In Table II, there are set forth the rate constants and aging rates for the 0.65 wt. palladium on 75/25 silica-alumina catalyst for temperatures and pressures within and without the desired range which shows that a maximum reaction rate and substantially zero aging rate are obtained within the shaded area of the drawing.

Table II 0.65% PALLADIUM ON 75/25 SILICA-ALUMINA CATALYST RATE CONSTANT K AND AGING RATE (-AY/l00 HRS.)

Pressure, p.s.i.a. Temperature F.

Hydrogen Hydro- 750 765 775 780 785 790 carbon using the previously-described impure n-pentane feed and the 0.65% wt. palladium on 87/ 13 silica-alumina catalyst.

The data in Table II also show that outside the range of conditions indicated by the shaded area of the drawing,

there is either excessive aging of the catalyst or an uneconomically low reaction rate for the process.

EXAMPLE III When palladium on silica-alumina catalysts are prepared using palladium concentrations in the range from 0.50 to 0.75% wt., it is found that there is relatively little change in catalyst activity with palladium concentration. If the rate constant K for formation of isopentane in a pentane isomerization process is plotted against palladium metal concentration in a palladium on silicaalumina catalyst, it is found that K increases linearly with the metal content in the range from zero to about 0.5% wt. palladium. K reaches a maximum at about 0.65-0.70% palladium and declines at higher palladium concentrations. While it has been found that the combination of conditions of temperature and hydrogen and hydrocarbon partial pressures is somewhat unique for a particular hydrocarbon feed containing a small amount of hydrocarbon impurities with respect to a particular catalyst composition, still the range of conditions required does not vary substantially for catalysts of substantially the same activity, particularly in the region of maximum activity. Therefore, catalysts in the range from 0.50 to 0.75% wt. palladium on silica-alumina, containing 70-90% silica, vary slightly in activity with metal content, but have an optimum reaction rate and substantially zero catalyst aging rate within the shaded area of the drawing. In Table III, there are set forth the reaction rate constants and catalyst aging rates for temperatures in the range from 750 to 790 F. and at different hydrogen and hydrocarbon partial pressures for catalyst consisting of 0.50% wt. palladium on 87/13 silica-alumina.

Table III RATE CONSTANT K AND AGING RATE (-AY/lOO HRS.) FOR 0.50% WT. PALLADIUM ON 87/13 SILIGA-ALUMINA Pressure, p.s.i.a. Temperature, F.

Hydrogen Hydrocarbon 750 775 785 790 In this table, it should be noted that the reaction rate constants and catalyst aging rates correspond to the values within the shaded area of the drawings where a maximum reaction rate is obtained and substantially zero aging rate.

EXAMPLE IV When palladium on silica-alumina catalysts are prepared in different sizes and in different palladium concentrations, it is found that there is a definite relationship between catalyst pellet size, palladium concentration, and resistance to aging. These catalysts increase in resistance to aging with decrease in catalyst pellet size and with increase in palladium concentration up to about 0.75% wt. Thus, catalyst pellets of A2" diameter are more resistant to aging than pellets of A" diameter, and A pellets (extruded and cut into short lengths, or formed by breaking A3" pellets in half) are even more resistant to aging. Similarly, catalysts containing more than 0.50% Wt. palladium are more resistant to aging than catalysts containing lesser amounts of palladium. The differences in aging due to size and palladium concentration are most apparent under conditions of temperature and pressure which are conducive to catalyst aging, the larger pellets having lower palladium content aging more rapidly. Under conditions Table IV Feed: 88% l1-C5Hm, 2% l C5H12, 5% cy-C5H1o, 5% n-CoHu Reaction conditions 780 F. 8.3 LWHSV H2-430 p.s.i.a. Hydrocarbon185 p.s.i.a. Catalyst support 7 5/25 silica-alumina Percent Particle Size, in. Aging Rate, Palladium AY/ hrs.

0.35 it pellets 5 0. 35 Lt extrudate 5 0.35 A0 crooked pellets 1.2 0.35 A6 extrudate 1. 2 0. 65 A6 extrudate 0. 5-1. 0

While I have described my invention with special emphasis upon one or more specific embodiments, I wish it to be understood that within the scope of the appended claims this invention may be practiced otherwise than as specifically described.

The embodiments of the invention in which an exclusive property or privilege is claimed. are defined as follows.

I claim:

1. A process for isomerization of a sulfur-free hydrocarbon feed comprising n-pentane containing :an appreciable amount of other C -C hydrocarbon as impurities, not exceeding about 10% vol. which comprises contacting said hydrocarbon feed and hydrogen with a catalyst consisting essentially of 0.50.75% wt. palladium on silicaaiumina, containing 7090% silica, at a hydrogen partial pressure of 375425 p.s.i.=a., and a hydrocarbon partial pressure of about -175 p.s.i.a. and reaction temperature of about 775790 F. lying within the shaded area of the drawing, said process being characterized by a reaction rate constant of about 8-12, which remains constant for periods of operation in excess of 100 hrs.

2. A process in accordance with claim 1 in which the principal hydrocarbon impurities are n-hexane and cyclopentane.

3. A process in accordance with claim 1 in which the hydrocarbon pressure and reaction temperature lie substantially on the line A--B in the drawing.

4. A process in accordance with claim 1 in which the catalyst support is 75 25 silica-alumina.

5. A process in accordance with claim 1 in which the catalyst support is 87/ 13 silica-alumina.

6. A process in accordance with claim 3 in which the catalyst comprises 0.65% wt. palladium on 87/13 silicaalumina.

7. A process in accordance with claim 6 in which the principal hydrocarbon impurities are n hexane and cyclopentane.

References (Iited in the file of this patent UNITED STATES PATENTS 2,550,531 Ciapetta Apr. 24, 1951 2,798,105 Heinemann etal. July 2, 1957 2,830,013 Northcott et a1. Apr. 8, 1958 2,831,908 Starnes et a1 Apr. 22, 1958 2,906,798 Starnes et al Sept. 29, 1959 2,925,453 Folkins et a1 Feb. 16, 1960 2,944,096 Teter et a1 July 5, 1960 FOREIGN PATENTS 487,392 Great Britain Oct. 21, 1952 i it 

1. A PROCESS FOR ISOMERIZATION OF A SULFUR-FREE HYDROCARBON FEED COMPRISING N-PENTANE CONTAINING AN APPRECIABLE AMOUNT OF OTHER C4-C7 HYDROCARBON AS IMPURITIES, NOT EXCEEDING ABOUT 10% VOL. WHICH COMPRISES CONTACTING SAID HYDROCARBON FEED AND HYDROGEN WITH A CATALYST CONSISTING ESSENTIALLY OF 0.5-0975% WT. PALLADIUM ON SILICAALUMINA, CONTAINING 70-90% SILICA, AT A HYDROGEN PARTIAL PRESSURE OF 375-425 P.S.I.A. AND HYDROCARBON PARTIAL PRESSURE OF ABOUT 130-175 P.S.I.A. AND REACTION TEMPERATURE OF ABOUT 775-790*F. LYING WITHIN THE SHADED AREA OF THE DRAWING, SAID PROCESS BEING CHARACTERIZED BY A REACTION RATE CONSTANT OF ABOUT 8-12, WHICH REMAINS CONSTANT FOR PERIODS OF OPERATION IN EXCESS OF 100 HRS. 